Twitter users over the weekend were the target of a scam that tried to infect them with rogue anti-virus software and other malware, in what is one of the first times the micro-blogging site has been hit by a known for-profit attack, a security researcher said.

The problem started after a flurry of tweets directed users to a website promising "Best Video." The site appeared to offer content from YouTube, but behind the scenes, the site delivered a PDF document designed to infect those using vulnerable versions of Adobe's Reader program. Victims then received an urgent warning that their systems were infected and needed to cleaned using fraudulent security software. More reading: